The present invention relates to high voltage semiconductor devices and particularly to high voltage semiconductor devices with an associated Zener diode capable of operating at higher voltages.
In high voltage semiconductor applications, elements such as a Zener diode are often integrated on one side of a semiconductor substrate on which a high voltage power semiconductor element having main electrodes on both faces is formed. A self-isolation type semiconductor device utilizes a p-n junction between the substrate region and the element region so that the element integrated on one face side of the semiconductor substrate is not affected by a voltage applied to the main electrode on another face side of the substrate.
FIG. 2 shows an example of a Zener diode portion of a self-isolation type semiconductor device. An n- bulk layer 3 is laminated on an n+ silicon substrate 1 on one face or side of the substrate, and an electrode 2 is adhered to the opposite side. A p- well 4 is formed selectively on the side of the substrate 1 of the n- layer 3. An n+ layer 5 is formed selectively on the side opposite to the substrate 1 of this p- well 4. This n+ layer 5 becomes the cathode region of the Zener diode, and the p- well 4 becomes the anode region thereof. A cathode electrode 8 is in contact with the n+ layer 5 at an opening of an insulating film 9. An anode electrode 7 is in contact with a p+ region 6 which surrounds the n+ region 5 and reaches the n- layer 3. By connecting the anode electrode 7 with an electrode of another integrated element and connecting the cathode electrode 8 with a terminal or still another integrated element, it is possible to have the Zener diode act as a protective element against an overvoltage. The high impurity concentration p+ regions 6 are useful as separating layers, and although they may be dispensed with, there is an effect to suppress the operation of parasitic elements produced between adjacent integrated elements.
FIG. 3 shows an example in which a bidirectional Zener diode is formed as a protective element against a bidirectional overvoltage. Two n+ cathode regions 5 are provided in the p- anode region 4, which are connected to other integrated elements or terminals by cathode electrodes 8. These two Zener diodes shown in FIG. 2 and FIG. 3 consisting of an anode region 4 and a cathode region 5 are separated from the rest of the semiconductor device by the p-n junction between n- layer 3 and the p- anode region 4 with respect to a positive voltage applied to the electrode 2.
When it is desired to increase the Zener breakdown voltage of the Zener diode shown in FIG. 2 or FIG. 3, the impurity concentration of the anode region 4 has to be lowered. However a parasitic bipolar transistor is formed by the n+ cathode layer 5 as an emitter, the p- well 4 as a base, and the n- bulk layer 3 and the n+ substrate 1 as a collector. When the concentration of the anode 4 is lowered, the d.c. current amplification factor h.sub.FE of this parasitic transistor increases. Therefore, the maximal voltage V.sub.CEO between the collector and the emitter is lowered. For example, V.sub.CEO is 90 to 95 V when the breakdown voltage of the Zener diode is at 5 V, and V.sub.CEO is at 80 to 85 V when the breakdown voltage is at 10 V. When the breakdown voltage is increased to 25 V, V.sub.CEO is lowered to 50 V. Since this V.sub.CEO determines the voltage applicable to the main electrode on the other side of the substrate, that is, the withstand or blocking voltage of this whole semiconductor device, the foregoing means that the withstand voltage of the whole semiconductor device is lowered as a result.
Objects of the present invention are to overcome such problems of the prior art and to provide a semiconductor device integrated with a Zener diode having a high breakdown voltage without lowering the withstand voltage of the whole semiconductor device.